How often/and for what are NEMLEC channels used for? I know what the organization is, but I don't know why they would have their own freqs. Especially with BAPERN being used for most interop

BAPERN is great for municipality to municipality communication, and is heavily used, however it does not have the capacity that NEMLEC provides. Every day there are surveillance operations, SWAT deployments, searches, testing, stings etc. that require communications. Each municipality could not afford the infrastructure required so a law enforcement council provides it.

In the beginning we had Intercity 158.9700 [Eastern Mass, RI and some southern NH]

In the 70's due to Riots, BAPERN was created. With BAPERN Came some other [little used / heavily used] UHF Regional NETS, these were
NEMLEC, Plym Cnty, Bristol Cnty and NESPERN [NorthEast aka Salisbury / Merrimac area].

With the extra fundings thru 9-11 and the need for more wide-area Interop, BAPERN is expanding [Area Wide 3 and 4 - formerly BAPERN 3 and 4] and have now included NON Law Enforcement onto the system.
BAPERN has added towers in Plymouth Cnty and Pending for Bristol Cnty

NEMLEC has merged some of it's Interop into the BAPERN System which is why the NEMLEC channel is part of the BAPERN scheme.